Cell and Molecular Biology 45: Bacterial Genetics (FLAG)

Why Study Bacterial Genetics

  • Molecular Biology and Genomics: Fundamentals of molecular mechanisms.
  • Human Health and Disease: Insights into pathogens and the human microbiome.
  • Bacterial Ecology: Understanding interactions within ecosystems.
  • Biotechnology Applications: Genetic engineering, antibiotic production, etc.

Timeline of Genetics

  • 1850: Early understandings of inheritance.
  • 1941: Beadle and Tatum: "One gene, one enzyme" hypothesis.
  • 1943: Luria Delbrück's work on bacterial inheritance.
  • 1953: Hershey & Chase demonstrated that DNA is the genetic material.
  • 1970s: Development of recombinant DNA technologies.
  • 1995: First genome of a free-living organism sequenced: Haemophilus influenzae.
  • 2003: Completion of the Human Genome Project.

Definition of Bacterial Genetics

  • Bacterial Genetics: Study of heredity mechanisms in bacteria, focusing on:
    • Chromosomes
    • Plasmids
    • Transposons
    • Phages
  • Techniques include culture, replica plating, mutagenesis, transformation, conjugation, transduction.

Bacterial Diversity

  • Bacteria comprise approximately 30% of Earth's DNA.
  • Tree of Life:
    • Bacteria: 74-76 phyla
    • Archaea: 24-28 phyla
    • Eukarya

Importance of Bacteria as Model Organisms

  • Haploid: Single chromosome copy, simplifying genetics studies.
  • Asexual Reproduction: Rapid and efficient proliferation.
  • Short Generation Time: E. coli can divide every 20 minutes.
  • Culture Conditions: Can grow in defined media (e.g., plates, broth).
  • Genetic Manipulation: Easy to create mutants and study gene functions.

Bacterial Genome Structure

  • Genome Characteristics:
    • Typically a single circular, double-stranded DNA chromosome.
    • Few exceptions, such as Borrelia burgdorferi.
    • Minimal inter-gene space; operons group functionally related genes.
    • Frequently carry plasmids (circular extrachromosomal DNA).

Bacterial Growth and Division

  • Binary Fission:
    • Asexual process of reproduction in bacteria.
    • Cell elongation occurs followed by DNA replication and division into two identical daughter cells.
  • Generation Time:
    • E. coli: ~20 min.
    • Clostridium perfringens: ~10 min (very fast).
    • Some bacteria divide extremely slowly, taking thousands of years.

Growth Requirements of E. coli

  • Capable of synthesizing cellular components from inorganic nutrients + energy source (e.g., glucose).
  • Minimal Medium Composition: Includes salts (e.g., K2HPO4, MgSO4), glucose, and trace metals.

Phenotypes in Bacterial Genetics

  • Wild-Type: Non-mutated strain exhibiting typical characteristics (prototroph).
  • Auxotrophic Mutants: Require additional nutrients for growth due to metabolic impairments.
    • Biosynthetic Auxotrophs: Require specific nutrients like amino acids.
    • Catabolic Auxotrophs: Cannot metabolize certain carbon sources.

Housekeeping Genes and Lethality

  • Essential genes for survival; mutations can result in lethal phenotypes (e.g., involved in DNA replication, glycolysis).
  • Conditional Lethal Mutants: Lethal under specific conditions (e.g., temperature).

Gene Nomenclature and Annotation

  • Gene Annotation:
    • Three lowercase letters for biochemical pathways + capital letter for the gene (e.g., leuB for the leucine pathway).
  • Gene vs. Protein Distinction:
    • Genes in lowercase (e.g., dnaA) vs. proteins in uppercase (e.g., DnaA).
  • Phenotypic Nomenclature:
    • Identifying phenotypes using the same mnemonic as genotypes but capitalized (e.g., Thr- for requiring threonine).

Nomenclature for Mutants and Pathways

  • Mutations: Change in the nucleic acid base sequence.
  • Alleles: Variants of genes that can affect metabolic pathways.
  • Components: Includes amino acids, carbon sources, vitamins, nucleotides, and resistance genes.

Summary of Key Terms in Bacterial Genetics

  • Wild type: Normal organism.
  • Mutant: Organism with a mutation compared to wild type.
  • Nomenclature is crucial for understanding genetic studies, involving specific letter codes for pathways and function-related gene products.